1. Field of the Invention
The present invention relates to cellular orthogonal frequency division multiplexing (OFDM) system, and more particularly, to a system and method for canceling co-channel interference (CCI) in OFDM system.
2. Description of the Related Art
OFDM system is considered as an approach that can satisfy high-speed, high-quality and high-capacity communications required in the fourth generation (4G) wireless communication system. OFDM signal has a plurality of subcarriers with a minimum interval in a frequency domain and data are transmitted in parallel. Therefore, OFDM system can reduce a data rate per subcarrier while maintaining a high overall data rate. In addition, OFDM scheme can provide high-speed, high-quality and high-capacity data communications by increasing the number of the subcarriers.
OFDM system is used efficiently in wireless systems under multipath fading environments. In addition, the use of the cyclic prefix (CP) eliminates the intersymbol interference (ISI) while maintaining the orthogonality between the subcarriers. Furthermore, the OFDM receiver is less complex than single carrier systems which apply equalizers with large number of taps or rake receivers.
To apply OFDM scheme to cellular communication system, frequency reuse is highly required to increase the spectrum efficiency of the system.
However, applying frequency reuse in the cellular communication system leads to CCI due to signals received from different cells. The CCI is considered as one of limiting factors of the overall system capacity. That is, when the frequency reuse is applied in the cellular OFDM network, mobile station (MS) receives, in addition to the desired base station (BS)'s signal, signals arriving from interfering BSs which use the same subcarrier frequencies. This leads to high degradation in the bit error rate (BER) performance and can lead, in severe interference situations, to link drop, i.e., outage, between MS and serving BS. In order to eliminate the CCI effect, CCI cancellers have been proposed. For example, various schemes employing minimum mean squared error (MMSE), MMSE coupled to serial interference canceller (SIC), MMSE coupled to parallel interference canceller (PIC), and maximum likelihood estimation (MLE) have been proposed to implement the CCI canceller.
Among these schemes, MLE scheme having the highest performance generates replicas of the received signals from candidates of the CCI signals and the desired signals. The replicas are generated from all possible weighted combinations of the desired signal and the CCI signals where the weights represent the estimated channels coefficients. The replicas are compared with the received signals. The replica with the minimum Euclidean distance is then selected and the data are detected.
In a conventional receiver with the CCI canceller employing the MLE scheme, the MLE CCI canceller can easily distinguish the desired signal from the interferer signals when the received power of the desired signal is different from that of the interferer signal. As a result, the detection error decreases. On the other hand, the data detection error rate of the MLE CCI canceller increases when the received power of the desired signal is nearly equal to that of the interferer signal under high interference environments such as cell edges. This is because several combinations of the signal candidates may generate similar replica with the minimum Euclidean distance. Therefore, when the received power of the desired signal is nearly equal to that of the interferer signal, the conventional MLE CCI canceller has difficulty in distinguishing the desired signal from the interferer signal.